Add matchBySku method to inventoryEntries

`client.inventoryEntries.all().matchBySku(['a', 'b', 'c', 'd']).fetch()`

The function is an helper to build a query predicate using `in` operator

`predicate = 'sku in ("a", "b", "c", "d")'`

See https://github.com/sphereio/sphere-stock-import/blob/master/src/coffee/stockimport.coffee#L223-L227